User's Manual

044-05117
Figure 4-1. Functional Block Diagram
4-4.1 Preamplifier
The RF carriers are applied to the input port of the amplifier, where they are fed to the
preamplifier stage. The preamplifier provides two stages of class-A mode-amplification. The
output of the preamplifier is then split into two paths, one to the main amplifier and one to the
error amplifier.
4-4.2 Main and Error Amplifiers
The main amplifier provides the balance of gain and power (refer to Table 1-1 for amplifier
specifications). The main amplifier employs class AB amplification for maximum efficiency. The
error amplifier and feed forward loops are used to correct signal distortion introduced by non-
linearity in the class AB main amplifier. The error amplifier operates in class A mode. The RF
signal from the preamp is coupled to an attenuator and phase shifter in the first feed-forward loop
where it is phase shifted by 180 degrees and amplified in the pre-main amplifier. The output from
the pre-main amplifier is fed to the class AB main amplifier. The signal output from the main
amplifier is sampled using a coupler, and the sample signal is combined with the main input
signal and input to the second feed-forward loop.
The error signal is attenuated, phase shifted 180 degrees, then fed to the error amplifier where it
is amplified to a level identical to the sample output from the main amplifier. The output from the
error amplifier is then coupled back and added to the output from the main amplifier. The control
loops continuously make adjustments to cancel out any distortion in the final output signals.
4-4.3 Alarm Monitoring and Control
The alarm logic controls the +5 Vdc bias voltage that shuts down the amplifier. During routine
operation, all normal variations are automatically compensated for by the feed-forward loop
control. However, when large variations occur beyond the adjustment range of the loop control, a
4-2 Installation and Service Manual - G3L-850-135 Multi-Carrier Power Amplifier